GroEL is a chaperonin from E. coli. It is essential for folding proteins in vivo. An initial crystal structure of GroEL has been determined by Sigler and co-workers in the absence of nucleotides. There are several ambiguities in this structure. We have grown several crystal forms of GroEL both in the presence and absence of nucleotides. We aim to resolve the ambiguities in the native structure and determine the structural changes produced by nucleotide binding.